In the production of hydrocarbons from subsurface formations, it is common to “hydraulically fracture” a formation in order to increase its permeability, which in turn enhances its productivity. Typically, a fracturing fluid is injected under pressure into the formation through an injection well. In order to determine the effectiveness of a fracturing operation, it is desirable to gain information about the rate and extent of fracturing that occurs during the injection.
In water flooding operations, fluids are injected into the subsurface formation to mobilize the hydrocarbons towards producer wells. Such injection often needs to occur at high pressures, occasionally exceeding the fracture pressure and leading to unintended fracturing of the formation and fluids being injected “out of zone.” This is an undesirable outcome that decreases the effectiveness of the water flood and may result in early water breakthrough at the producers or water entering other producing intervals or overlying formations. Thus, it is desirable to gain information about the rate and extent of fracturing that occurs during the water flooding.
Various techniques have been proposed for monitoring fracturing. One such technique uses passive seismic monitoring that depends on detection of the microseismic signals that result from hydraulic fracturing, as recorded in nearby observation wells or in the treatment well itself. This process provides information in real-time and depends on the existence, strength, and detectability of the microseismic signals. Thus, it remains desirable to provide a robust and inexpensive technique that gives accurate and meaningful information about fracture growth without these limitations.